Apparatus for unidirectionally solidifying metals

ABSTRACT

A pair of annular heat pipes are arranged end-to-end around a casting mold. One of the heat pipes is heated above the liquidus temperature of the alloy to be cast, while the other heat pipe is heated below the solidus temperature of the alloy. A chill is placed at one end of the mold to cool the alloy unidirectionally. A sharp temperature gradient that is virtually a step function is produced at the junction of the two heat pipes which results in greater control over the region of solidification of the alloy as the heat pipes are moved to cool and solidify the alloy.

United States Patent 11 1 Kirkpatrick et al. Nov. 6, 1973 [54] APPARATUSFOR UNIDIRECTIONALLY 3,651,240 3/1972 Kirkpatrick 13/1 X SOLIDIFYING-METALS 2,214,976 9/1940 Stockbarger 23/273 3,317,958 5/1967 Stroup etal. 165/64 Inventors: Milton Kirkpatrick, Palos Verdes, 2,508,988 5 1950Bradley 165/30 Calif.; Thomas S. Piwonka, North g i f Bruce Marcus LosPrimary Examiner-J. Spencer Overholser nge e l Assistant Examiner.lohnE. Roethel [73] Assignee: TRW, Redondo Beach, Calif. Att0rneyDaniel T.Anderson et al. [22] Filed: Jan. 10, 1972 21 Appl. No.: 216,736 [571ABSTRACT Related Application Data A pair of annular heat pipes arearranged end-to-end [63] Continuation of Ser No 34 415 Ma 4 1970 arounda casting mold. One of the heat pipes is heated abandoned y above theliquidus temperature of the alloy to be cast, while the other heat pipeis heated below the solidus 52 us. c1 164/338 164/60 165/105 temperature0f the A is Placed end 51 lm. c1. 822d 27/04 13226 25/06 of the moldunidirecmnaw A Sharp [58] Field of Search 164/60 2 165/30 temperaturegradient that is virtually a step function is 65/64 produced at thejunction of the two heat pipes which results in greater control over theregion of solidifi- [56] References Cited cation of the alloy as theheat pipes are moved to cool UNITED STATES PATENTS and Smidify the3,532,155 10/1970 Kane et a] 164/60 8 Claims, 2 Drawing Figures PATENTEUNM 6 I973 Milton E, Kirkpatrick Thomas S.Piwonku Bruce 0 MarcusINVENTORS AGENT ,1 APPARATUS FOR UNIDIRECTIONALLY SOLIDIFYING METALSThis is a continuation of application Ser. No. 34,415, filed May 4,1970, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to the art of metal casting and more particularly to improvedapparatus for undirectionally solidifying metals, especially metalalloys.

2. Description of the Prior Art It is known in the metal casting artthat if the mold is heated above the melting point of the metal beingcast, except at one end where it is chilled, the heat will flow from themetal in the direction of the chilled end and the metal will solidifyunidirectionally. The resulting metallurgical structure of the metalwill have grains or crystals that are likewise aligned in the directionwhere heat flow occurred. Certain properties of unidirectionallysolidified metals are better than those that are solidifiedconventionally. For example, unidirectionally solidified castings areused to make improved turbine blades. Furthermore, if a eutectic alloyis undirectionally solidified, the resulting structure will be anaturally produced composite of aligned needles or platelets boundtogether within a matrix of bare metal.

In certain cases, it is desirable that the metal casting be in the formof a single crystal. This is also a desirable structure for turbineblades. To achieve this type of metallurgical structure, it is necessaryin the metal cooling process that a very steep thermal gradient bemaintained across the liquid-solid interface as it moves from the coldend to the hot end. A principal object of this invention is to increasethe thermal gradient acting upon the liquid-solid interface of a metalsubjected to unidirectional solidification.

SUMMARYOF THE INVENTION is arranged end-to-end with the first one andwith means for establishing within the second heat pipe a lower uniformtemperature below the solidus temperature of the alloy.

A mold is placed within the first higher temperature heat pipe, with acooling means or chill located near the junction of the two heat pipes.Means are provided for moving the two heat pipes together along thelongitudinal axis so that the mold effectively moves out of the hotterheat pipe and into the cooler one. Because the heat pipes aresubstantially isothermal along their lengths, each with differenttemperature, the temperature gradient at their junction is rather steepand approaches a step function. The thermal gradient is considerablysharper than that produced by conventional means.

BRIEF DESCRIPTION OF THE DRAWING baffle plate 44 may, in addition,

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thereis shown a rigid base 10 and an upstanding elongated support member 12affixed thereto. The top of the support member 12 supports a chill blockor plate 14 of thermally conductive material, such as copper. The chillblock 14 is preferably provided with internal passageways through whichextend a series of conduits 16 for conducting a cooling fluid. Thecoolant may be supplied from a source 18 which pumps the fluid throughthe conduits 16 between an inlet tube 20 and an outlet tube 22.

On top of the chill block 14 is supported a ceramic mold 24 which may beopen at both ends. The mold 24 has the shape of the desired article tobe cast, such as turbine blade.

In accordance with the invention, the mold 24 is surrounded by twoannular heat pipes, one being a high temperature heat pipe 26 and theother a low temperature heat pipe 28. Both heat pipes 26 and 28 are aslong as the mold 24 and are joined end to end by a suitable connectingmeans 30. The heat pipes 26 and 28 may have a similar geometricconfiguration, such as the same cross-section, be it circular, square,or rectangular, and may have substantially the same length. Thus, theymay have the same symmetry about a longitudinal axis 32 and have acombined length about double that of the mold 24.

The heat pipes 26 and 28 are movable vertically along the longitudinalaxis 32. Longitudinal movement may be provided by mounting the lowerheat pipe 28 on a platform 34 and raising or lowering the platform 34along a guide rod 36 by means of a driven screw 38. As will be explainedmore fully below, the heat pieps 26 and 28 must be moved a sufficientdistance to raise the upper heat pipe 26 from an initial position whereit entirely encloses the mold 24 to a second position where the mold 24is entirely removed therefrom.

An annular radiation baffle plate 40 is mounted between the adjacentends of the two heat pipes 26 and 28. The baffle plate 40 is made ofrefractory material or high melting point metal, such as molybdenum toprotect the chill block 14 from the high temperature of the upper heatpipe 28 and to aid in sharpening the thermal interfacebetween the twoheat pipes 26 and.

28. The baffle plate 40 is formed with a central opening 42 large enoughto pass the mold 24 when the heat pipes 26 and 28 are moved vertically.

An additional radiation baffle plate 44 is mounted on top of the upperheat pipe 26, closing the opening thereof to prevent loss of heat fromthe internal regions of the upper heat pipe 26. The radiation baffleplate 44 may comprise a solid plate as shown, in which case it wouldneed to be moved topermit the charging of the mold 24 withmolten metal.Alternatively, the baffle plate 44 would not require movement prior tocharging the mold 24 if it is provided with a central opening throughwhich the molten metal can be poured. The be independently heated to aidinheating the mold 24. In this case, the battle plate 44 may itself be aheat pipe.

A thermocouple 46 extending through the upper baffle plate 44 monitorsthe temperature of the upper heat pipe 26. The upper heat pipe 26 isheated by an induction coil 48 which receives energy from a source 50 ofradio frequency current. The induction coil 48 may comprise only a fewturns encompassing a small portion of the length of the heat pipe 26.The reason for this is that the heat pipe is such an efficient thermalconductor that, if heat is applied to only a small area thereof, theheat is readily transmitted over the entire volume, and when thermalequilibrium is reached, it becomes an isothermal surface. As analternative to induction heating, electrical resistance heating or anyother suitable form of easily controllable heat may be used. Whateverform of heating energy is used, it must be sufficient to heat the upperheat pipe 26 and maintain it at the operating temperature for which itis designed, which must be a temperature above the liquidus temperatureof the alloy to be cast. For some of the nickel base super alloys, theliquidus temperature is around 2500F.

The lower heat pipe 28 is similarly provided with a heater coil 52 thatreceives radio frequency energy from a source 54. The heating energysupplied to the lower heat pipe 28 must be sufficient to heat it andmaintain it at the operating temperature for which it is designed, whichin this case is a temperature below the solidus temperature of the alloybeing cast, say below 2000F for the nickel base super alloys. The heatercoil 52 is spaced a short distance from the lower end of the lower heatpipe 28 so as to accommodate a cooling means such as a coiled tube 56.The cooling tube 56 is supplied coolant fluid from a source 58. Thepurpose of the cooling tube 56 is to remove excess heat from the castingmold as the lower heat pipe 28 is raised along the length of mold 24,thereby tending to keep heat pipe 28 below the solidus temperature ofthe alloy being cast.

Each of the heat pipes 26 and 28 is annular in configuration and ispreferably one of the kind disclosed in the copending application ofMilton E. Kirkpatrick, Ser. No. 797,725 filed Jan. 31, 1969, entitledHeat Transfer Device. The details of construction of only the upper heatpipe 26 will now be described, it being understood that the constructionof the lower heat pipe 28 will be similar.

Referring now to FIG. 2, the heat pipe 26 includes concentric inner andouter cylindrical metal tubes 62 and 64, respectively. The space betweenthe tubes 62 and forms an annular chamber 60. The surfaces of the tubes62 and 64 disposed within the annular chamber 60 are covered withlinings 66 and 68 or porous wick material. The two wick linings 66 and68 are spaced apart and joined together by short spacer elements 70 ofwick material that are spaced along the length of the tubes 62 and 64. 3

The annular chamber 60 is closed at both ends of ring-like cover plates72, which leave an isothermal working space 73 open within the heat pipe26 for easy access from the outside. The annular chamber 60 is evacuatedof non-condensable gases, such as air, and contains a vaporizableworking fluid 74 of sufficient quantity to wet the entire wick materialby capillary action. The specific fluid depends upon the operatingtemperature desired for the heat pipe 26.

The wick material for the linings 66 and 68 and spacer elements 70 maybe in the form of sintered metal, wire screens, or other porous compactshaving voids or openings of capillary size and capable of transportingthe vaporizable working fluid 74.

In the operation of the heat pipe 26, the heater coil 48 is energized toheat the portion of the annular heat pipe 26 surrounded thereby. Theworking fluid 74 heated thereby vaporizes and the vapor carries awayfrom the high heat flux region thermal energy equivalent to the heat ofvaporization. The vapor migrates through the annular chamber where itcondenses on all interior surfaces that are below the temperature of thevaporizing surface, thereby giving up the heat of vaporization to andraising the temperature of all the cooler surfaces. Continuous vaporflow paths are provided along the annular extent of the annular chamber70 by means of the radial and linear spacing between the spacer elements30. The condensed working fluid 74 is then transported by capillaryaction through the V wick material from these condensing regions to thevaporizing region or'high heat flux input zone, where the working fluid74 again vaporizes.

By means of this closed loop process, thermal energy supplied by theheater coil 48 is transported and delivered to any and all coolerinterior regions of the cham-' ber 70. The result is that the entiresurface of the heat pipe 26 quickly becomes an isothermal surface whenoperating in the temperature range determined by the working fluid, andthe volume within the isothermal working space 73 is uniform intemperature along the entire length of the heat pipe 26.

For a specific working fluid, there is a range of equilibriumtemperatures over which the annular heat pipe will provide isothermalconditions. The lower limit of the equilibrium temperature range isdetermined by the thermodynamic properties of the working fluid, namelythe vapor pressure and the heat of vaporization. The upper limit of theequilibrium temperature range is determined by the mechanical ability of,the device to withstand the positive pressures of the vapor relative tothe surrounding atmosphere.

For casting the nickel base super alloys referred to above, the upper orhigh temperature heat pipe 26 and the wicks therefor may be fabricatedfrom a columbium bare alloy such as 99 percent columbium and 1 percentzirconium, or alloy C-lO3 containing 89 percent columbium, 10 percenthafnium, and 1 percent titanium. The working fluid may be lithium. Forthe lower or low temperature heat pipe 28 and the wicks therefor, thematerial may bejstainless steel and the working fluid may be sodium.

The operation of the metal casting apparatus will now be described. Withthe heat pipes 26 and 28 and ceramic mold 24 in the position shown inFIG. 1, the

heater coils 48 and 52 are energized. When the heat pipes 26 and 28 havereached their respective isothermal equilibrium temperatures and themold 24 has reached the same temperature, the upper baffle plate 44 isremoved and molten metal or alloy is poured into the mold 24. Uponstriking the chill block 14, the molten metal immediately solidifiesas alayer and forms a seal between the chill block 14 and the mold 24 sothat the molten metal poured on top of the solidified metal layer willnot leak out. When the mold 24 is filled with molten metal, the upperbaffle plate 44 is replaced.

With the mold 24 and heat pipes 26 and 28 in their relative positionsshown in FIG. 1, the upper heat pipe 26 is an isothermal surface havinga temperature above the liquidus temperature of the molten metal. Theceramic mold 24 and the molten metal therewithin are subjected to thatuniform temperature along their vertical length, and they would also beat that same uniform temperature except for the presence of the chillblock 14 which subjects the base of the mold 24 and the solidified metallayer to a much lower temperature. A thermal gradient is immediatelyestablished in the molten metal, which tends to conform to the sharpthermal gradient established outside the mold 24. Externally of the mold24, the sharpness of the thermal gradient is due to the uniformisothermal temperature of the upper heat pipe 26 which subjects the mold24 to heat by radiation and the abrupt drop in temperature at the chillblock 14 which extracts heat from the mold and metal by conduction.

Thus, in the initial stages of metal solidification, the metal issubjected to cooling preferentially in the direction of the chill block14, or to unidirectional cooling in that direction. This gives rise tothe formation and growth of crystals or grains that are oriented in thevertical direction. in the mold 24, the metal is partly solidified nextto the chill block 14, the bulk of it is molten above the liquidustemperature, and at the interface of the molten and solid there is anintermediate zone that is partly liquid and partly solid where thegrains or crystals are forming.

The lower heat pipe 28 is maintained as an isothermal surface at atemperature below the solidus temperature of the metal. At the junctionof the two heat pipes 26 and 28 there is a sharp thermal gradient equalto the difference in isothennal temperatures of the heat pipes 26 and28. I

in order to solidify the molten metal, the heat pipes 26 and 28 arejointly moved vertically upwards to subject the mold 24 and metaltherein to a moving sharp thermal gradient. As the heat pipes 26 and 28move up the length of the mold 24, the mold 24 loses by thermalradiation the thermal energy it received from the upper heat pipe 26. Nolonger heated by the upper heat pipe 26, the cast metal transfers itsheat to the chill block 14 by conduction or by radiation to the lowerheat pipe 28 which moves into the areas of the mold 24 previouslyoccupied by the upper heat pipe 26. As the metal is solidified, thelower heat pipe 28 provides cooling in the radial direction of the metalcasting lying below the liquid-solid interface. However, radial coolingof the metal after it has solidified does not affect the unidirectionalmetallurgical structure that was established as the metal solidified inthe region of the sharp thermal gradient.

The rate at which the heat pipes 26 and 28 traverse the mold 24 may beadjusted for each specific alloy to provide unidirectional growth of thesolidified metal as well as economy in processing time.

When the heat pipes 26 and 28 have traversed the entire length of themold 24, the power to the heater coils 48 and 52 may be shut off and themold allowed to cool further to the ambient temperature. The coolingprovided by the coiled tube 56, as well as the conductive coolingprovided by the chill plate 14, contribute to the cooling process whichterminates at or near ambient temperature.

The upper and lower heater coils 48 and 52 may be attached to theirrespective heat pipes 26 and 28 so that they move along with them.Alternatively, they may remain fixed while the heat pipes move, providedthey are properly located to heat the pipes in all positions. The coiledtube 56' is preferably fixed to the lower end of the lower heat pipe 28.

Some of the alloys which are used for turbine blades are as follows:

Alloy Nominal Composition Melting Range F M252 0.16C, 0.02Mn, 0.08Si,19.1Cr,

9.95Co, 9.7Mo, 2.5Ti, l.lAl,

0.06Zr,'2.lFe, Balance Ni 2450 2500 lN-lOO O.l8C, lOCr, I5Co, 3M0,5.5Al,

5Ti, 1.0V, 0.0SZr, 0.0158,

Balance Ni 2300 2400 U-700 0.15C, 3.25Al, 3.5Ti, 5.1Mo, l5Cr,

18.5Co, 1.0Fe, 0.1B, Balance Ni 2200 2550 lnconel 7l3 0.14C, 0.25Mn,0.5Si, l3Cr,

9.5Mo, 0.75Ti, 6.0Al, 2.5Fe,

2.3Cb-l-Ta, Balance Ni 2300 350 It has been found that, for best resultswith the above alloys, the intermediate mushy zone (the two phase regionwhere solid and liquid coexist) between the liquid and solid metalshould be kept to a minimum. The thermal gradient thereby desired, whichis a few hundred degrees per inch for the above alloys, is easilyachieved by means of the invention.

We claim:

1. In combination:

a. first and second annular heat pipes joined together in closeend-to-end spaced apart adjacency and having central passagewayssubstantially aligned along a common axis;

b. means including first and second heater means coupled separately tosaid first and second heat pipes respectively for establishingisothermal conditions in said heat pipes along said axis, with saidfirst heat pipe at a higher equilibrium temperature than said secondheat pipe, thereby to establish along said axis a temperature profilecharacterized by a step function;

e. means for supporting a workpiece within the central passagewaydefined by said first heat pipe;

. means providing relative motion between said workpiece and said heatpipes along said axis so as to cause said workpiece to move out of thepassageway of said first heat pipe into the passageway of said secondheat pipe, thereby causing a thermal gradient represented by said stepfunction to traverse the length of said workpiece; and

e. cooling means coupling to said second heat pipe at a location spacedlongitudinally from said second heater means and controllable to removeexcess heat from said workpiece when the workpiece is within the workingspace of said second heat pipe, thereby to minimize any reduction in themagnitude of said thermal gradient.

2. The invention according to claim I, wherein said,

heat pipes are provided with different working fluids having differingvaporizing temperatures;

and further wherein said heater means comprise means for heating saidheat pipes to the vaporizing temperatures of their respective workingfluids.

I 3. The invention according to claim 2, and further in cluding meansfor cooling said workpiece by conduction in a direction along said axis.

4. Metal casting apparatus, comprising:

a. a first annular heat pipe having a central tubular working spacearranged along a longitudinal axis and provided with means forestablishing and maintaining in said first heat pipe a uniformtemperature above the liquidus temperature of a desired alloy to besolidified from the molten state;

b. a second annular heat pipe joined with said first heat pipe in closeend-to-end spaced apart adjaceney and having a central tubular workingspace aligned along said longitudinal axis and provided with meansincluding heater means for establishing and maintaining in said secondheat pipe a uniform temperature below the solidus temperature of saiddesired alloy to thereby establish a sharp thermal gradient at thejunction of said heat pipes;

c. a mold disposed within the working space of said first annular heatpipe for receiving a charge of said alloy in liquid form;

d. means for extracting heat from a portion of said mold along adirection parallel to said longitudinal axis;

e. means for providing relative motion between said mold and said heatpipes to cause said mold to move from one working space to the other andthereby cause the junction where said thermal gradient occurs totraverse said mold from a position adjacent to said heat extractingmeans to the opposite end thereof in a direction along said longitudinalaxis; and

f. cooling means coupled to said second annular heat pipe at a locationspaced longitudinally from said heater means and controllable to removeexcess heat from said mold when said mold is within the working space ofsaid second annular heat pipe, thereby to maintain said second annularheat pipe below the solidus temperature of said alloy and therebyminimize any reduction in the magnitude of said thermal gradient.

5. The invention according to claim 4, wherein said heat pipes areprovided with different working fluids having differing operatingtemperatures,vand further including additional heater means for heatingsaid first heat pipe to the operating temperature of its respectiveworking fluid.

6. The invention according to claim 5, wherein said heater meanscomprises an electrical coil wound around a portion of each of said heatpipes respectively.

7. The invention according to claim 6, wherein said heat extractingmeans comprises a chill block upon which said mold is supported.

8. The invention according to claim 4 wherein said cooling meanscomprises a tube coiled around a portion of said second heat pipe, andfurther including means for causing coolant fluid to flow through saidcoiled tube.

1. In combination: a. first and second annular heat pipes joinedtogether in close end-to-end spaced apart adjacency and having centralpassageways substantially aligned along a common axis; b. meansincluding first and second heater means coupled separately to said firstand second heat pipes respectively for establishing isothermalconditions in said heat pipes along said axis, with said first heat pipeat a higher equilibrium temperature than said second heat pipe, therebyto establish along said axis a temperature profile characterized by astep function; c. means for supporting a workpiece within the centralpassageway defined by said first heat pipe; d. means providing relativemotion between said workpiece and said heat pipes along said axis so asto cause said workpiece to move out of the passageway of said first heatpipe into the passageway of said second heat pipe, thereby causing athermal gradient represented by said step function to traverse thelength of said workpiece; and e. cooling means coupling to said secondheat pipe at a location spaced longitudinally from said second heatermeans and controllable to remove excess heat from said workpiece whenthe workpiece is within the working space of said second heat pipe,thereby to minimize any reduction in the magnitude of said thermalgradient.
 2. The invention according to claim 1, wherein said heat pipesare provided with different working fluids having differing vaporizingtemperatures; and further wherein said heater means comprise means forheating said heat pipes to the vaporizing temperatures of theirrespective working fluids.
 3. The invention according to claim 2, andfurther including means for cooling said workpiece by conduction in adirection along said axis.
 4. Metal casting apparatus, comprising: a. afirst annular heat pipe having a central tubular working space arrangedalong a longitudinal axis and provided with means for establishing andmaintaining in said first heat pipe a uniform temperature above theliquidus temperature of a desired alloy to be solidified from the moltenstate; b. a second annular heat pipe joined with said first heat pipe inclose end-to-end spaced apart adjacency and having a central tubularworking space aligned along said longitudinal axis and provided withmeans including heater means for establishing and maintaining in saidsecond heat pipe a uniform temperature below the solidus temperature ofsaid desired alloy to thereby establish a sharp thermal gradient at thejunction of said heat pipes; c. a mold disposed within the working spaceof said first annular heat pipe for receiving a charge of said alloy inliquid form; d. means for extracting heat from a portion of said moldalong a direction parallel to said longitudinal axis; e. means forproviding relative motion between said mold and said heat pipes to causesaid mold to move from one working space to the other and thereby causethe junction where said thermal gradient occurs to traverse said moldfrom a position adjacent to said heat extracting means to the oppositeend thereof in a direction along said longitudinal axis; and f. coolingmeans coupled to said second annular heat pipe at a location spacedlongitudinally from said heater means and controllable to remove excessheat from said mold when said mold is within the working space of saidsecond annular heat pipe, thereby to maintain said second annular heatpipe below the solidus temperature of said alloy and thereby minimizeany reduction in the magnitude of said thermal gradient.
 5. Theinvention according to claim 4, wherein said heat pipes are providedwith different working fluids having differing operating temperatures,and further including additional heater means for heating said firstheat pipe to the operating temperature of its respective working fluid.6. The invention according to claim 5, wherein said heater meanscomprises an electrical coil wound around a portion of each of said heatpipes respectively.
 7. The invention according to claim 6, wherein saidheat extracting means comprises a chill block upon which said mold issupported.
 8. The invention according to claim 4 wherein said coolingmeans comprises a tube coiled around a portion of said second heat pipe,and further including means for causing coolant fluid to flow throughsaid coiled tube.